1. Field of the Invention
The present invention relates to an image formation apparatus having charging means for coming into contact with and charging an image carrying member, and developing agent charging means for charging a developing agent on the image carrying member.
2. Description of the Related Art
Hitherto, there have been known image formation apparatuses such as photocopiers, printers, facsimile apparatuses, and so forth, using transfer-type electrophotography or electrostatic recording. Electrophotography image formation apparatuses comprise: a photosensitive member serving as an image carrying member which is normally a rotating drum; charging means for uniformly charging the photosensitive member to a predetermined polarity and potential (i.e., performing a charging step); electrostatic latent image formation means, i.e., exposing means for forming an electrostatic latent image on the charged photosensitive member (i.e., performing an exposing step); developing means for manifesting the electrostatic latent image formed on the photosensitive member with a developing agent (toner) to form a developing agent image (toner image) on the photosensitive member (i.e., performing a developing step); transfer means for transferring the toner image from the surface of the photosensitive member onto a recording medium such as paper or the like (i.e., performing a transfer step); cleaning means for removing any residual toner remaining on the photosensitive member following the transfer step to clean the surface of the photosensitive member (i.e., performing a cleaning step); fixing means for fixing the toner image on the recording medium (i.e., performing a fixing step); and so forth. The photosensitive member is repeatedly used in the image formation processing made up of the steps for charging, exposing, developing, transfer, and cleaning.
Following the toner image having been transferred onto the recording medium, the toner remaining on the surface of the photosensitive member is removed therefrom by a cleaning device serving as cleaning means. The removed toner is recovered in the cleaning device and stored as waste toner. However, it should be noted that the amount of waste toner generated should be as little as possible, from the perspective of the environment and effective use of resources. This has led to development of a known image formation apparatus wherein the waste toner collected in the cleaning device is returned to the developing means, and re-used there.
There are also known cleaner-less type image formation apparatuses wherein the cleaning device has been done away with. With cleaner-less type image formation apparatuses, the toner remaining on the photosensitive member following the transferring process is removed by the developing means, and the removed toner is recovered within the developing means and re-used there.
The cleaner-less type image formation apparatuses which clean the toner remaining on the photosensitive member in this way perform cleaning simultaneously with the process for developing the electrostatic latent images with the developing means, which we will refer to as “simultaneous developing and cleaning”.
Simultaneous developing and cleaning is a method for recovering the residual toner in the developing means, that is, the transfer-residual developing agent (transfer-residual toner) remaining on the photosensitive member without being transferred onto the recording medium by the transfer means, at a subsequent developing step.
That is to say, the photosensitive member with toner remaining continues to be charged by the charging means, an electrostatic latent image is formed on the surface of the photosensitive member by the exposing means, and an electrostatic latent image is developed thereupon by the developing means. Then, simultaneously with the electrostatic latent image being developed by the developing means, the residual toner at the unexposed portions without being developed is recovered by a bias for removing fogging, which is a fogging-removal potential difference Vback, more specifically, a potential difference between the DC voltage applied to the developing means and the surface potential of the photosensitive member.
According to this method, the residual toner is recovered by the developing means, and re-used in subsequent developing processing. Accordingly, there is no waste toner generated, also doing away with troublesome maintenance for recovering the waste toner. Further, the image formation apparatus can be reduced in size since there is no need for the cleaning device.
However, even in arrangements wherein the charging means is contact-charging means wherein the photosensitive member is charged by the contact means coming into contact with the surface of the photosensitive member, of the residual toner on the photosensitive member, the toner having a charge with opposite polarity to the regular polarity of the toner (i.e., inverted toner) adheres to the contact charging means in particular, at the time of the residual toner on the photosensitive member passing through a contact nip portion (charging portion) between the photosensitive member and the contact charging means. Accordingly, the contact charging means is contaminated by the toner beyond a permissible level, to the extent that the changing means cannot charge the photosensitive member sufficiently.
Now, a cleaner-less type image formation apparatus such as described above has been proposed wherein adhesion of residual toner to the charging means, in the event that the charging means is contact charging means, is prevented, with substandard charging and images being done away with by the developing means effectively collecting the residual toner, and further wherein the advantages of the cleaner-less method are maximized.
Such an image formation apparatus comprises first developing agent charging means positioned downstream in the direction of rotation of the photosensitive member from the transfer means but upstream from the charging means for forming the electrostatic latent image, for charging the residual developing agent on the photosensitive member, and second developing agent charging means positioned downstream form the first developing agent charging means but upstream form the charging means, for successively charging the residual developing agent on the photosensitive member.
The first developing agent charging means charges the residual developing agent, i.e., the developing agent remaining on the photosensitive member without being transferred by the transfer device, to a polarity opposite to the regular polarity of the developing agent (toner). Next, the second developing agent charging means charges the developing agent which has been charged to a polarity opposite to the regular polarity of the residual developing agent (toner), to the regular polarity. Subsequently, the charging means charges the photosensitive member, and at the same time uniformly charges the residual toner as appropriate.
Due to this, adhesion of the residual toner to the charging means is prevented, and the developing means can effectively recover the residual developing agent, thereby doing away with substandard charging and images, and further maximizing the advantages of the cleaner-less method.
However, there have been the following problems in non-steady operation states, such as upon start-up or closing down of the image formation apparatus.
First, in the event of bringing the first or second developing agent charging means into contact with the photosensitive member, there is some toner which remains on the developing agent charging means. This toner loses its force to remain at the nip between the photosensitive member and the developing agent charging means in the instant that bias is applied to the developing agent charging means or in the instant that application of bias is stopped, and accordingly is discharged onto the photosensitive member. The amount of charge of such toner is not controlled by the developing agent charging means, the toner adheres to the charging means at the time of being transferred to the charging means according with the movement of the photosensitive member, and accordingly leads to substandard charging or substandard images.
Second, the charging potential on the photosensitive member is unstable at the time of starting up or ending image formation actions, and a predictable potential is not available. Particularly, in the event that magnetic brushes or the like are used for the developing means, toner may adhere to the photosensitive member depending on the potential thereof even in the event that there is no supply of electric power to the developing means for example, or in the event that two-component developing is being employed, the carrier may adhere to the photosensitive member depending on the potential thereof, leading to substandard images.